1. Field of the Invention
The present invention relates to a surface light source device used in a non-emission display device or the like, and more particularly, relates to a surface light source device using point sources, such as light emission diodes (LED).
2. Description of the Related Art
There has been proposed a non-emission display device as represented by a liquid crystal display device. This non-emission display device is provided, behind a liquid crystal display device, with a surface light source device (backlight unit) for illuminating it.
Regarding such a surface light source device, Japanese Utility Model Registration No. 2578529 discloses a surface light source device having an optical waveguide that is adapted so as to take light from a light source through a side portion (incident surface) of the optical waveguide and to radiate the light as illumination light from a front portion (exit surface) of the optical waveguide. Due to this optical route, the above surface light source device is referred to as “edge-light type surface source device”.
This surface light source device has a plurality of light sources that are arranged so as to oppose the side portion (incident surface) of the optical waveguide. Additionally, light emitted from the optical waveguide toward its rear side is reflected by a reflective sheet and brought back into the optical waveguide again.
In the “edge-light type” surface source device mentioned above, the light source is formed by either a linear light source (e.g. cold cathode fluorescent lamp) or point sources [e.g. light emission diodes (LED)]. With progress of luminous efficiency, it has been recently noted to employ light emission diodes as the point sources. The reasons are as follows: the light emission diode is generally longer lasting; it is effective in miniaturization and weight-saving; and the light emission diode is made from mercury-free materials, causing less deterioration of environment.
In adopting a point source (e.g. a light emission diode) as the light source, a plurality of point sources (e.g. a light emission diodes) are arranged in a straight line separately from each other. Additionally, these light emission diodes are arranged so as to oppose the side portion (incident surface) of the optical waveguide. In the surface light source device disclosed in Japanese Utility Model Registration No. 2578529, additionally, the optical waveguide is provided, on the side portion (incident surface), with recesses for receiving the light emission diodes in view of facilitating positioning of the diodes in the optical waveguide.
Japanese Patent Publication Laid-open No. H10(1998)-82915 discloses another surface light source device having an optical waveguide that is adapted so as to take light from a light source through a rear portion (incident surface) of the waveguide and to radiate the light as illumination light from a front portion (exit surface) of the waveguide. Due to this optical route, the above surface light source device is referred to as “inland-type surface source device”. This surface light source device has a plurality of light sources that are arranged so as to oppose the rear portion of the optical waveguide. Additionally, light emitted from the optical waveguide toward the rear side of the optical waveguide is reflected by a reflective sheet and brought back into the optical waveguide again.
In this inland-type surface source device, similarly, the light source is formed by either a linear light source (e.g. cold cathode fluorescent lamp) or point sources (e.g. light emission diodes). In adopting a point source (light emission diode) as the light source, a plurality of point sources are arranged in a plane separately from each other. Additionally, these point sources are arranged so as to oppose the rear portion (incident surface) of the optical waveguide. In the surface light source device disclosed in Japanese Patent Publication Laid-open No. H10(1998)-82915, additionally, the optical waveguide is provided, on the rear portion (incident surface), with recesses for receiving the light emission diodes in view of facilitating positioning of the diodes in the optical waveguide.
In common with the “edge-light” type surface source device and the “inland” type surface source device, there are accordingly arranged, ahead of a front portion of the optical waveguide, a lens sheet for collecting lights emitted from the optical waveguide in view of improving luminance, and a diffuser sheet for uniformizing the luminance.
In the above-mentioned surface light source devices each having a plurality of point sources (light emission diodes) as the light source juxtaposed, however, the illumination light radiated from the exit surface of the optical waveguide is easy to cause luminance unevenness in connection with positioning of the point sources. The longer a distance between one point source and its adjoining point source gets, the more remarkable the luminance unevenness becomes.
Thus, if reducing the number of point sources with an elongation of the distance between the neighboring point sources in the above-mentioned surface light source device, then the above-mentioned luminance unevenness is caused to make nonsense of various merits brought by reducing the number of point sources (merits: e.g. easiness in production, reduction in manufacturing cost).
Meanwhile, if the surface light source device adopts, as the light source, light emission diodes each emitting a monochromatic light of red, green, or blue, it is required to make a high-grade incandescent light as a result of mixing respective colored lights emitted from these light emission diodes. Thus, the surface light source device has to equip an optical waveguide having a sufficient thickness and a light mixing chamber having a wide space so that lights emitted from the light emission diodes for respective colors are mixed with each other sufficiently.
In the above-mentioned “inland” type surface source device, an adoption of such an optical waveguide and a light mixing chamber having a wide space would cause the whole surface source device to be large-sized in thickness. Additionally, if the optical waveguide having a sufficient thickness is made from plastic material, then light loss is increased in an interior of the optical waveguide and at its boundary surfaces with surrounding materials. It means that the surface source device has to equip a number of light emission diodes, making both easiness in production and reduction in manufacturing cost even harder. Further, the above-mentioned conventional surface light source device is not provided under consideration of realizing a high-grade incandescent light as a result of mixing red, green, and blue lights with each other.